Windowed bot-visit totals grouped by identified agent.

Backs the hub Traffic page's "By agent" module. Same scoping as every other impressions read here: owned content only, general crawlers excluded (_ai_visit_filters), and unattached visits (no content_item_id) invisible via the inner join.

SUM(visit_count), never COUNT(*) — Cloudflare ingestion rows are hour-buckets carrying counts > 1. NULL identified_llm folds into "unidentified" so the shares always total 1.0. Rolling window (now - days), not completed-days: this is a composition readout, not a chart, so the partial-day bias the series endpoints avoid doesn't apply.